Homeworld series is all about epic space battles, in an RTS style.
There's Freelancer, a fairly open rpg with fun space dogfights, some getting quite massive, a game I really liked back in the day (or it's predecessor StarLancer, focused only on the dogfights. Fights are very difficult if you don't have a joystick, but very rewarding and fun if you do)
There's Sol Exodus, trying to do a similar thing as StarLancer, not a bad game, modern control scheme.
There's Space Pirates And Zombies, an indie open rpg-ish/rts-ish game, you might like it, personally I found it kinda boring
And, of course you can't get more epic, massive, whatever than EVE Online. That is kinda the ultimate space experience if you're willing to sink the time in an MMO. It's also kinda the only MMO out there that has no resemblance to WoW, everything is player-run, you can get to own corporations with thousands of people, the game world is... insanely huge, probably well over a thousand systems by now (each with it's own planets, stations, asteroid belts, etc) and you can pilot a ship a few meters long to a Titan class a few Kilometers long. Did I mention the space battles with thousands of players at a time?
What's best is that there are no different realms, or server clusters. Everybody is playing on the same realm at the same time. You can get 50.000 players online at the same time in the same game world.